This invention relates to a nickel-base superalloy article having a surface region of reduced carbon content and an aluminum-containing protective layer deposited on the surface.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1800-2100xc2x0 F.
Many approaches have been used to protect the turbine blades and vanes against the highly aggressive combustion-gas environment and to increase the operating temperature limit of the turbine blades and vanes. For example, the composition and processing of the base materials themselves have been improved. Physical cooling techniques may also be used.
In another approach, the surfaces of the turbine blades and vanes are coated with aluminum-containing protective coatings that protect the articles against the combustion gas, and in some cases insulate the articles from the temperature of the combustion gas. The articles are thereby able to run cooler and be more resistant to environmental attack.
There may be chemical interactions between the article substrate and the protective coating. In some cases, these interactions adversely affect properties, such as in the formation of topologically close packed (TCP) phases and secondary reaction zones (SRZ). In other cases, the interactions have beneficial effects, such as in the diffusion of strengthening elements from the article substrate into the protective coating. It has been observed that the diffusion of reactive elements such as hafnium and zirconium from the substrate into the coating improves the thermal cycling performance of the coating. However, those results have not always been consistent, and there is a large scatter in the data. Thus, even though there has been an indication of improved performance as a result of the diffusion effects, those improvements cannot be relied upon in all cases.
There is a need for an approach to improving the results obtained by the diffusion of reactive elements from the substrate of the nickel-base superalloy article into the protective coating. The present invention fulfills this need, and further provides related advantages.
The present invention provides a procedure that improves the performance of aluminum-containing protective coatings applied to the surface of a nickel-base superalloy having high levels of reactive elements such as hafnium, zirconium, yttrium, lanthanum, and cerium, and an article having this improved performance. The procedure is readily performed with available apparatus, and may be integrated into the coating process. The coating protects the surface of the article against environmental damage, as in the case of conventional protective coatings.
A method for preparing a surface-protected nickel-base superalloy article comprises the step of providing an article substrate having a surface and having a nominal bulk composition comprising the nickel-base superalloy. The nickel-base superalloy comprises more nickel than any other element, a reactive element selected from the group consisting of hafnium, zirconium, yttrium, lanthanum, and cerium, and combinations thereof, and a nominal bulk composition of carbon. The method further comprises depositing a protective layer overlying the surface of the article substrate. The step of depositing a protective layer includes steps of decarburizing locations, such as surface regions, where the carbon serves as a barrier to the diffusion of the reactive element from the substrate into the protective layer, and depositing an aluminum-containing protective layer overlying the substrate.
In a typical case, the reactive element is hafnium present in an amount of more than about 0.20 weight percent (preferably more than about 0.5 weight percent) in the nominal bulk composition, or zirconium present in an amount of more than about 0.05 weight percent in the nominal bulk composition. The carbon content in the nominal bulk composition is more than about 0.05 weight percent, and the carbon content of a decarburized surface region is less than about 0.02 weight percent, averaged through the surface region. The surface region preferably has a thickness of from about 5 micrometers to about 100 micrometers.
In practicing the method, the reducing of the carbon content is preferably accomplished by contacting a decarburizing agent to the surface of the substrate, decarburizing a platinum-containing layer after deposition (where the protective layer is a platinum aluminide), depositing the aluminum-containing layer from an atmosphere containing a decarburizing agent, and/or decarburizing the substrate and protective layer after it is deposited. The decarburizing agent is preferably a reducing gas such as hydrogen or carbon dioxide.
The present approach provides a low-carbon region in the substrate, adjacent to the surface where the protective layer is deposited, and in the protective layer. The lower carbon content in this region reduces the tendency to form carbides of the reactive elements. Such carbides reduce the level of the free, unreacted reactive element that is available to diffuse to the protective layer and improve its properties. The result is improved performance of the coating during service.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.